1. Field of the Invention
The invention relates to solid oxide cells (SOCs) in which the formation of degradation products is reduced. More specifically the invention concerns solid oxide fuel cells and solid oxide electrolysis cells in which degradation of oxygen electrodes comprising lanthanum-strontium-manganite (LSM) is inhibited.
2. Description of the Related Art
Solid oxide cells, also known as reversible solid oxide cells, can be used as solid oxide fuel cells and as solid oxide electrolysis cells. The solid oxide cell basically consists of three different layers—a middle layer of an oxide ion conducting electrolyte that is gastight that is sandwiched between electrode layers. The electrode layers are porous, electron and ion conducting and each solid oxide cell has an oxygen electrode and a fuel electrode. A solid oxide fuel cell is described in the following:
A solid oxide fuel cell (SOFC) is a high temperature fuel cell which generates electricity directly from an electrochemical reaction, and it is composed entirely of solid-state oxide materials, typically ceramics. This composition allows SOFCs to operate at much higher temperatures than other fuel cell types such as PEM fuel cells. Typical operating temperatures are 600° C. to 1000° C.
In the solid oxide fuel cell the oxygen electrode is the cathode where a reduction of oxygen to oxygen ions takes place. The fuel electrode is the anode where oxidation of hydrogen to hydrogen ions and then water takes place. An electrochemical energy conversion takes place in the solid oxide fuel cell, whereby electricity is generated from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react therefore at the electrodes in the presence of an electrolyte.
Usually the reactant flowing to the anode is a fuel such as hydrogen or methane. When methane is used as a fuel, internal reforming takes place at the anode surface whereby methane is reformed in the presence of steam to hydrogen and carbon monoxide. The hydrogen is then converted in the electrochemical reaction. The oxidant flowing to the cathode is usually air or oxygen.
Solid oxide fuel cells can be operated in reverse mode as solid oxide electrolysis cells (SOEC) to perform electrolysis of H2O and/or CO2 for hydrogen or synthesis gas (a mixture of hydrogen, H2 and carbon monoxide, CO) production.
In the solid oxide electrolysis cell the oxygen electrode is the anode where an oxidation of oxygen ions to oxygen takes place. The fuel electrode is the cathode where reduction of water to hydrogen takes place.
Conventional composite oxygen electrodes are manufactured using an electron conductive material such as lanthanum-strontium-manganite (LSM) and an oxygen ion conductive material such as yttria-stabilised zirconia (YSZ). These oxygen electrodes are deposited on a dense electrolyte surface made of an oxygen ion conductive solid oxide such as YSZ.
The reduction-oxidation reactions take place mainly at the triple phase boundaries where the electrode, the electrolyte and oxygen or hydrogen are in contact with each other. The triple phase boundary is therefore influenced by the reactions occurring at the interface between the electrode and the electrolyte. Efficient gas diffusion and increased contact areas between the electrolyte and the electrodes are therefore important.
Performance of the oxygen electrode is mainly determined by the resistance present at the oxygen electrode-electrolyte interface. It is desirable to reduce the interfacial resistance and increase the occurrence of triple phase boundaries. Thereby electrode polarisation resistance is decreased and the overall performance of the oxygen electrode is improved. The cell operation temperature and the partial pressure of oxygen in the oxygen electrode chamber also influence the performance of the oxygen electrode. Inadequate control of the above factors can lead to the formation of degradation products at the triple phase boundaries and thus a reduction in the SOFC or SOEC performance.
A known degradation product resulting in increased interfacial resistance between the oxygen electrode and the electrolyte is lanthanum zirconate, La2Zr2O7 (abbreviated LZO). This undesirable degradation product is formed at the interface between the lanthanum-strontium-manganite (LSM) oxygen electrode and the yttria stabilised zirconia (YSZ) electrolyte and its formation is increased by heating of the for instance SOFC at high temperatures such as during sintering of the SOFC. The formation of LZO is also increased under high polarisation during cell testing.
Other known degradation products are strontium zirconate (SZO) and oxide compounds of La—Zr—Si, and Sr—Zr—Si.
Degradation of LSM-YSZ has been identified to be the dominant contribution to cell degradation under severe test conditions i.e. at low temperatures and high current densities. Barfod et al., Solid State Electrochemistry Proc. 26th Risø, International Symposium on Materials Science, Risø National Laboratory, Roskilde, page 121 (2005), also point out that the degradation rate is strongly dependent on the oxygen partial pressure on the cathode side in solid oxide fuel cells, the degradation rate being significantly higher in air than in pure oxygen.
Both LZO and La-silicate have been reported by D. Kuscer et al., Solid State Ionics 78 (1995) 79, to form at a LaMnO3/YSZ interface during aging at 1450° C.
Studies (A. Hagen et al., Electrochemical Society Transactions, Vol. 2007-07, No. 1, page 301-309) indicate that LZO is present as nano-sized particles distributed locally and preferably in LSM/electrolyte contact areas in long term tested solid oxide fuel cells. The formation of nano-sized strontium zirconate (SZO) particles at the interface between the cathode and electrolyte may also be a possibility. Both LZO and SZO phases have insulating properties due to their low conductivity when compared to zirconia electrolyte, and their presence weakens the electrical contact of the cathode and electrolyte.
Various attempts have been made to inhibit the formation of lanthanum zirconate. U.S. Pat. No. 7,141,329 B2 discloses an electrode having a microstructure of extended triple phase boundary with a porous ion conductive ceria film coating. This coating is made from one or more doped oxide sols selected from CeO2 polymeric sols or particulate sol and can be manufactured at a lower temperature by employing a sol-gel method resulting in preventing generation of undesired interfacial reaction products.
F. Umemura et al. (Denki Kagaku Oyobi Kogyo Butsuri Kagaku (Electrochemistry and Industrial Physical Chemistry) (Japan) v63:2. (5 Feb. 1995) page 128-133) evaluated the microscopical characterization of a degraded air electrode to examine the sintering and reaction of an electrode material. Half cells were produced and measured by La0.9Sr0.1MnO3 and 8YSZ obtained when 8% Y2O3-stabilized zirconia was added. Adding the YSZ controlled the generation of La2Zr2O7 in the interface between the electrode and electrolyte.
The electrochemical characteristics of La1-xSrxMnO3 for solid oxide fuel cells and the formation of lanthanum zirconate has also been studied by H. M. Lee in Materials Chemistry and Physics, 2003, V77, N3 (January 30), page 639-646. The optimum amount of Sr in La1-xSrxMnO3 for a solid oxide fuel cell cathode material was studied by observing the charge transfer resistance, electrical conductivity, and reactivity with the electrolyte. The reactivity between the electrolyte and La1-xSrxBO3 (B=Cr, Mn, Co) was investigated and it was found that the secondary phase, La2Zr2O7, was not formed when the substitution amount of Sr was 50 moles.
Other known attempts to control the formation of LZO and SZO include utilising in the SOFC a LSM cathode containing a surplus of Mn. During preparation the LSM cathode is super stoichiometric in manganese, whereby manganese oxide is present as a secondary phase in the cathode.
JP patent application no. 5190183 discloses a solid oxide fuel cell with a fuel electrode containing yttria stabilised zirconia. Subsequently a slurry of powdered yttria stabilised zirconia powder and MnOx powder is made and this slurry is applied to the surface of the solid electrolyte, followed by calcination. Manganese thus exists in the three-phase zone consisting of fuel electrode, a solid electrolyte and the gaseous phase. The activation polarisation of the fuel electrode became smaller and output of the SOFC cell improved.
The chemical reactivity and interdiffusion of LSM and YSZ have been studied by J. A. M. Roosmalen, Solid State Ionics 52 (1992) page 303-312. They observe the formation of reaction products LZO and SZO and propose that resulting reaction layers including LZO and SZO might result in both ohmic and polarisation losses of the SOFC. They suggest that the ohmic losses are due to the low conductivity of the reaction products and the polarisation losses are due to the blocking of oxygen transfer at the three-phase boundary between cathode, electrolyte and oxygen. It is suggested to reduce the La and/or the Sr activity by decreasing the (La, Sr):Mn ratio in LSM.
However, these steps of introducing manganese in a super stoichiometric amount, though shown effective in reducing zirconate formation, are not enough to avoid the formation of LZO and SZO in the interface in the triple phase boundaries between the oxygen electrode comprising LSM and the YSZ electrolyte in solid oxide cells, when these are operated for prolonged periods and at strong polarisation.